In these days, many people are carrying devices such as personal digital assistants (PDAs), mobile phones, and medical equipment with them all the time. As a signal transfer method for transferring various data between those devices, there are a wired transmission method using a cable, and a wireless transmission method using, e.g., electric waves or light.
The wired transmission has advantages of high data security and high transmission speed, but is inconvenient because a user must carry a wiring device such as a cable all the time.
The wireless transmission is convenient in data transmission but has a limitation in that an economical price cannot be achieved because of needs for additional circuits for wireless transmission.
To overcome such a limitation, human body communication is being proposed, which uses a human body as a transmission medium. In the human body communication, a signal output through a transmitter of a communication device is applied to the human body through an electrode connected to the human body, and then is received by a receiver of another communication device through another electrode connected to the human body. The human body communication is convenient in use because no wiring device such as a cable is required, and can achieve an economical price because additional circuits for wireless transmission are not necessary.
FIG. 1 illustrates signal transfer in the human body communication. In the drawing, reference numbers 11, 12 and 10 indicate a signal transmitter, a signal receiver, and a human body, respectively.
A signal output from the signal transmitter 11 is applied to the human body 10, and is transmitted to the signal received through the human body 10. Through this signal transfer process, the human body communication is performed. Even if the signal transmitter 11 and the signal receiver 12 are individually provided in FIG. 1, they can be provided together in a dual human-body communication system.
FIGS. 2 and 3 illustrate influence of a ground electrode according to locations of a signal transmitter/receiver.
FIG. 2 shows signal loss decrement according to contact of a ground electrode with the human body at wrist and fingertip locations of the signal transmitter 11 in the case where the signal receiver 12 is placed at the right wrist and a corresponding ground electrode does not contact the human body. FIG. 3 shows signal loss decrement according to contact of a ground electrode with a human body at wrist and fingertip locations of the signal receiver 12 in the case where the signal transmitter 11 is placed at the left wrist and a corresponding ground electrode does not contact the human body. Each of the graphs according to the location of the signal transmitter/receiver represents a difference between a signal loss value when the ground electrode contacts the human body and a signal loss value when the ground electrode does not contact the human body.
Electrodes constituting a communication device for human body communication are divided into a signal electrode and a ground electrode according to their functions. The signal electrode is connected to an output signal line of a transmitter of the communication device or an input signal line of a receiver of the communication device, and serves to transmit/receive signal to/from the human body. The ground electrode is connected to a ground of the communication device. The characteristic of signal loss through the human body varies according whether the ground electrode contacts the human body.
To observe a signal-loss change according to the contact of the ground electrode with the human body, the decrement of the signal loss by contact of the ground electrode with the human body is experimented, and the experimental results are shown in FIGS. 2 and 3. The positive (+) decrement of the signal loss means that the signal loss decreases by the ground electrode, while the negative (−) decrement of the signal loss means that the signal loss increases.
FIG. 2 illustrates signal loss variations according to the contact of the ground electrode with the human body in the case where the signal transmitter 11 is placed at the left wrist and fingertip. Here, the signal receiver 12 is placed at the right wrist and a corresponding ground electrode does not contact the human body.
If the signal transmitter 11 is placed at the wrist 201, the signal loss decreases as the ground electrode of the signal transmitter 11 contacts the human body. In contrast, if the signal transmitter 11 is placed at the fingertip 202, the signal loss does not change or rather increases because of the ground electrode.
This is because the amount of electromagnetic field coupling of the ground electrode through the human body varies according to the location (wrist or fingertip) of the signal transmitter.
In more detail, if the signal transmitter 11 is placed at the wrist, an area of the human body contacting the ground electrode increases because of the relatively large area of the wrist, thereby increasing the amount of electromagnetic field coupling through the human body. Consequently, the signal loss decreases. However, if the signal transmitter 11 is placed at the fingertip, the area of the human body contacting the ground electrode is small because of a relatively small area of the fingertip, thereby reducing the amount of coupling. Consequently, the signal loss increases.
FIG. 3 illustrates variations of the signal loss by contact of the ground electrode with the human body with respect to the signal receiver 12, showing cases where the signal receiver 12 is placed at the right wrist 211 and the right fingertip 212, respectively. Here, the signal transmitter 11 is placed at the left wrist, and a corresponding ground electrode does not contact the human body.
As shown in FIG. 3, experimental results with respect to the signal receiver 12 are similar to experimental results with respect to the signal transmitter 11 as shown in FIG. 2. The cause of this phenomenon is also similar to the case of the signal transmitter 11 of FIG. 2.
From the experimental results of FIGS. 2 and 3, if the signal transmitter 11 or the signal receiver 12 for the human body communication is placed at the wrist (see 201 and 211 of FIGS. 2 and 3), the respective corresponding ground electrodes must contact the human body in order to reduce the signal loss.
However, if the signal transmitter 11 or the signal receiver 12 is placed at the fingertip (see 202 and 212 of FIGS. 2 and 3), the respectively corresponding ground electrodes should not contact the human body. The reason thereof will now be described.
The first reason is to prevent an increase in signal loss caused by the ground electrode. As described above, in the case where the signal transmitter 11 or the signal receiver 12 is placed at the fingertip, the contact of the ground electrode with the human body increases the signal loss.
The second reason is to reduce the amount of current consumed while flowing between a ground electrode and a signal electrode. The contact of the ground electrode with the human body causes a considerably large amount of current to flow between the signal electrode and the ground electrode through the human body. For example, in the case where the ground electrode contacts the human body in a band of about 5 MHz, if a voltage of about 3V is applied, a current of about 16 mA flows between the signal electrode and the ground electrode. In contrast, if the ground electrode does not contact the human body under the same condition, a current of about 0.2 mA flows. Accordingly, since the contact of the ground electrode with the human body does not reduce the signal loss or rather increases the signal loss, the amount of current being consumed can be reduced by brining the ground electrode out of contact with the human body. Accordingly, power being consumed by the signal transmitter 11 or the signal receiver 12 can be reduced.
As described above, signal loss and power consumption caused by the ground electrodes vary according to a human body portion, e.g., a wrist or a fingertip, which the signal transmitter/receiver 11/12 for human body communication contacts. For this reason, contacting the ground electrode with the human body all the time is very inefficient in terms of signal loss and power consumption.